How are avionics technicians certificated?

Given that I hold a valid FCC General Radiotelephone
license, I presume I can do my own panel work providing..

1. Any structural or weight and balance issue is
signed off by an A or A&P rated mechanic.

2. Any required calibration is done by a shop having
the proper instruments and factory training.

Nope, you presume wrong. The work must be done in an
FAA approved repair facility. Your general phone only
qualifies you to work in such a facility.